SQL & PL/SQL

I've been spinning my wheels on this and figured I'd give the forums a try.

I need to find the 13th largest value over the previous 52 weeks. I can set the window size correctly using ROWS 52 PRECEDING AND CURRENT ROW ORDER BY WEEK, but then I need another sort for the nTH largest within that partition. NTH_VALUE is close but what I really need is NTH_LARGEST_VALUE. Excel has a LARGE() function to do this.

Any thoughts for achieving this? See the example below:

Column desired_output is what I'm looking to achieve. It is the 2nd largest value over the current row and the previous 2 rows (ordered by week). Assume no relationship between week and val. These values just used for illustration purposes.

with tab as 
(
    select to_date('4-jan-2012', 'dd-mon-yyyy')+(level-1)*7 week, 
           to_number(to_char(to_date('4-jan-2012', 'dd-mon-yyyy')+(level-1)*7,'dd')) val,
           case when level in (1,2) then null
                when level = 3 then 11
                when level = 4 then 18
                when level = 5 then 18
                when level = 6 then 8
                when level = 7 then 8
                when level = 8 then 15
                when level = 9 then 22
                when level = 10 then 22
           end desired_result
      from dual connect by level <= 10
) 
select week, val, desired_result
  from tab 
order by week;  

WEEK             VAL DESIRED_RESULT
--------- ---------- --------------
04-JAN-12          4
11-JAN-12         11
18-JAN-12         18             11
25-JAN-12         25             18
01-FEB-12          1             18
08-FEB-12          8              8
15-FEB-12         15              8
22-FEB-12         22             15
29-FEB-12         29             22
07-MAR-12          7             22

10 rows selected.

Any advice would be appreciated. Thanks!